His arrest came during a police search of an address in Boxgrove Park, Guildford.

A post-mortem examination carried out on Thursday proved inconclusive, and the Independent Police Complaints Commission is now investgating the circumstances surrounding Mr McLean's death following a referral by Surrey Police.

IPCC commissioner Jennifer Izekor said: "This is a tragic loss of a young life and my thoughts are with Mr McLean’s family at this difficult time.

“We have met Mr McLean’s family to explain our role, outline the investigation to them and as much as possible answer their questions at this early stage.

“I would like to assure Mr McLean’s family we are independent of the police and our investigation will piece together evidence in order to build up a picture of the circumstances surrounding his death.”

The IPCC said Mr McLean became "seriously unwell" while under arrest in Boxgrove Park, and was then "visibly unwell" at the police station.

White powder

During the search of the premises a second man, aged 40 - who had also been detained - collapsed as well.

He was treated by an ambulance crew before being taken to hospital. He recovered and was then taken into custody at Staines police station.

Both men were thought to have taken an unknown substance. A white powder was seized at the property and is being analysed.

West Surrey Neighbourhood Superintendent Duncan Greenhalgh said: “Drug use and dealing in any form will not be tolerated in Surrey.

“We are working closely with local partners and other agencies to raise awareness among substance users of the risks associated with taking drugs.”

Anyone with any information about drug dealing is asked to call Surrey Police on 101, or contact Crimestoppers anonymously on 0800 555111.